Abstract Antibody subclass has been critically linked to variable potency in the anti-viral activity of polyclonal antibodies: variation in IgG subclass switching has been associated with variable vaccine efficacy; and in natural infection, subclass switching has been associated with variable viral suppression. Furthermore, in a number of studies, subclass switched variants of bnAbs have strongly modulated not only their effector function via tuning of interactions with Fc receptors, but neutralization breadth and potency. These observations suggest that antibody subclass may be a key parameter of antibody activity in vivo?contributing both to enhanced viral recognition, neutralization capacity, as well as innate immune recognition via Fc receptors, suggesting that better understanding of both the signals associated with and the significance of subclass switching will contribute to the development of a protective HIV vaccine. We hypothesize that IgG subclass composition contributes significantly to antiviral protection and can be rationally modulated by vaccination. We propose a 3- pronged approach to generalize our understanding of the significance of subclass switching to protections from HIV infection. First, we will evaluate diverse monoclonal HIV-specific antibodies across a variety of in vitro anti- viral antibody activities in IgG1, IgG2, IgG3, and IgG4 forms. Second, we will evaluate whether the unique IgG3 hinge topology imparts a general enhancement in neutralization potency by comparing the activity of monovalent Fab and bivalent Fab'2 fragments from polyclonal donor serum. Lastly, we will compare the efficacy of subclass switched monoclonal antibodies in in vivo viral challenge experiments in a humanized mouse model. Additionally, the signals associated with subclass-specific class switch recombination in human B cells are poorly defined in comparison with mouse B cells. A greater understanding of the stimuli and their downstream molecular regulators that promote IgG3 responses would help guide the design of vaccine formulations that would bias HIV responses towards IgG subclasses that are protective and away from those that are ineffective or detrimental. To determine the mechanisms controlling IgG subclass selection, first, we will identify and characterize the cytokines and co-stimulatory signals that promote optimal class switching to IgG3 in purified B cell cultures. Second, we will specifically as well as systematically target intracellular regulators of class switch recombination to and away from IgG3.